Medium-chain TAGs

The free FA and MAG are absorbed by the enterocytes of the intestinal wall and absorbed Upids are transported in water-soluble form to other tissues. FA with chain lengths shorter than 14 carbon atoms are bound to albumin and preferentially transported directly to the liver via the portal vein. Only a smaU proportion of MCFA undergoes a conversion to LCFA and esterified to TAG. A very small fraction of LCFA is transported via the portal route. This fraction increases when long-chain TAGs are fed in combination with medium-chain TAGs. The absorbed hpid fractions consist of FA, 2-MAG, some 1-MAG, lyso PL, some PL, fat-soluble vitamins, and small amounts of glycerol and cholesterol. The first step in mucosal transport is reesterification, and the second step is the synthesis of transport particles the so-called lipoprotein (chylomicron) and very low-density lipoproteins (VLDL). They enter the bloodstream via the lymph vessels. Lipoprotein hpase located on the interior walls of the capillary blood vessels hydrolyzes the TAG, releasing FA. These enter... 

In contrast, medium-chain fatty acids are mainly but not exclusively taken up as FFA whatever their position in medium-chain or mixed-chain TAGs. 

FAs liberated from food during absorption are metabolized more easily if they are short or medium chain, i.e., C10 or below. The sn-2 monoacylglycerols can be absorbed directly. Therefore, essential or desired FAs are most efficiently utilized from the sn-2 position in acylglycerols. In accordance with this, TAGs with short-chain FAs (SCFAs) or MCFAs at the sn-1 and sn-3 positions and PEFAs at the sn-2 position are rapidly hydrolyzed with pancreatic lipase (sn-1,3-specific lipase) and absorbed efficiently into mucosal cells. SCFAs or MCFAs are used as a source of rapid energy for infants and patients with fat malabsorption-related diseases. LCFAs, especially DHA and arachidonic acid, are important in both the growth and development of an infant, while n-3 PEFAs have been associated with reduced risk of cardiovascular disease in adults (Christensen et al., 1995 Jensen et al., 1995). 

TAG containing PUFA at the 2-position and medium-chain acyl groups at the 1- and 3-positions may be a useful nutraceutical, providing the benefits described above for Salatrim, Caprenin, and Caprucin , with the added benefit of an essential FFA being adsorbed by the body. Such a structured TAG has been suggested as a nutrition source for patients with pancreatic deficiencies. However, a problem recently discovered for structured TAG prepared by 1,3-selective lipases is the product s relatively low oxidative stability.l ... 

FIGURE 13.9 Production of MLM-type TAGs through transesterification with medium-chain FA or its ethyl ester using immobilized 1,3-position-specific lipase. 

In addition, because 1,3-position-speciflc lipases act on PUFAs weakly, all FAs at the 1,3-positions cannot be exchanged with medium-chain FAs. It is therefore difficult to produce high-purity MLM-type TAGs by transesterification of natural oils with immobilized 1,3-position-specific lipases. 

FA distribution in TAG as well as in phospholipids affects the physical properties, lipolitic and oxidative stability, and nutritional availability of lipids. In many TAG, the FA are arranged in a nonrandom distribution. In plants, monoenoic FA and PUFA are dominant at a sn-2 position (Orthoefer, 1996). In pig depot fat and in cow s milk, the TAG sn-2 position is occupied by palmitic acid. The distribution is also different in cattle and sheep depot fats (Love, 1996). In blubber seals, long-chain n-3 PUFA are esterified rather in sn-l,3 positions, whereas, in muscle, TAG in the sn-2 position that is typical for the lipid muscles of nearly all fish (Ackman, 1994). Enzymatic syntheses of structured TAG containing dY-5,8,ll,14,17-eicosapentaenoic acid (EPA) and dY-4,7,10,13,16,19-docosahexaenoic acid (DHA) in the sn-2 position with medium-chain FA at the end positions are particularly interesting (Halldorsson... 

Mu, H., and Hoy, C.E. (2002) Distribution of Medium Chain FA in Different Lipid Classes After Administration of Specific Structured TAG in Rats, Lipids 37, 329-331. 

Structured lipids (SL) are TAG that contain combinations of short-chain fatty acids (SCFA), medium-chain fatty acids (MCFA), and long-chain fatty acids (LCFA) located in the same glycerol molecule these may be produced by chemical or enzymatic processes (29,30). These specialty lipids may be produced via direct esterification, acidolysis, and hydrolysis or interesterification. Structured lipids are developed to fully optimize the benefits of their fatty acid varieties in order to affect metabolic parameters such as immune function, nitrogen balance, and lipid clearance from the bloodstream. 

Fig. 5. Diacylglycerol (DAG) stimulates mRNA production of enzymes for p-oxidation in the small intestine of C57BL/6J mice. C57BL/6J male mice, 7 wk old, were fed 5% triacylglyc-erols (TAG low-fat control), 30% TAG + 13% sucrose (high-fat control), 15% DAG + 15% TAG -I- 13% sucrose for 10 d (before obesity development). Values are means SEM, n = 6. P< 0.01, p< 0.001. Abbreviations AGO, acyl-CoA oxidase MCAD, medium-chain acyl-CoA dehydrogenase UCP, uncoupling protein-2 FAT, fatty acid translocase L-FABP, liver fatty acid binding protein.

L = long chain fatty acids, M = medium chain fatty acids, S = short chain fatty acids, MAG = monoacylglycerols, DAG = diacylglycerols, TAG = triacylglycerols. Substrate specificity to acylglycerols. 

A study involving two types of stmctured lipids (randomized v.s. specific product) revealed that the fatty acids located in the sn-2 position of TAG are preferentially absorbed (10). The randomized stmctured lipid used contained EPA, DHA and capric acid randomly distributed in the molecule. The specific stmctured lipid had EPA and DHA at the sn-2 position and capric acid at the sn-1 and snA positions of TAG. The concentrations of EPA and DHA in lyn5)hatics were higher in the specific stmctured lipid than in the randomized stmctured lipid. Rapid hydrolysis and absorption of stmctured lipids containing medium-chain fatty acids at the snA and snA positions and long-chain fatty acids at the sn-2 position have also been reported (II). 

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